In the machining of some specialty workpieces, for instance a crank shaft, it is necessary to fit the ends of the workpiece into chucks at locations offset radially from the axis of rotation of the chucks. With a crankshaft it is necessary to hold the shaft by its ends even when machining the shaft portions that the piston rods engage, these shaft portions being parallel to but offset from the shaft ends.
The chuck for such a machining operation typically has a chuck body that is rotated about the chuck axis and a head that can be moved on the body perpendicular to the chuck axis to adjust for the eccentricity of the workpiece. This head carries an abutment peg and two jaws that can be moved relative to the abutment peg to clamp the workpiece to it. Thus the head is set so that the abutment peg is spaced the desired distance from the clamping axis, the workpiece is held on the peg, and the jaws are actuated to clamp the workpiece.
The disadvantage of this arrangement is that the part of the workpiece being clamped can vary somewhat in diameter. This variation, even if the part is perfectly on center, will be reflected in the radial offset of the rotation axis from the clamp axis. In other words the distance between the clamp and the rotation axes is determined by the position of the head on the chuck body as well as by the diameter of the workpiece part being clamped. If the part diameter changes significantly it is necessary to reset the head, even if the same offset is desired between the clamp and rotation axes.